WAN Edge—MPLS VPN over DMVPN—2547oDMVPN (Hub and Spoke Only)

DMVPN provides two key advantages for extending MPLS VPNs to the branches, bulk encryption and, more importantly, a scalable overlay model. Since the assumption here is that the branches in this deployment are connected to the hub through a Layer 3 SP service, a tunneled model using GRE is needed to extend MPLS to the branches. Coupled with the fact that there is large number of existing DMVPN deployments, this solution becomes an attractive deployment option.

DMVPN allows the hub to have a single multipoint GRE tunnel interface to support large numbers of spokes. The spokes can be point-to-point or multipoint GRE tunnels depending on the requirement of direct spoke-to-spoke communication. Spoke-to-Spoke Communication (via Hub) discusses the advantages of point-to-point GRE tunnels at the spokes in the context of the current implementation of MPLS VPN over DMVPN.

To seamlessly extend the enterprise MPLS/Layer 3VPN MAN network to the remote branches, the WAN edge router (also the DMVPN hub in this case) should be a P device to label switching packets between the hub and the branches. As shown in Figure 6-1, the WAN hub router acts as a MPLS/Layer 3VPN P router to establish the LDP neighbor relationship and label switch packet with branch routers which act as a MPL3/Layer 3VPN PE router. The single IGP process is running on the entire enterprise MAN/WAN network to enable the branch routers to establish the MP-iBGP session with RRs in the enterprise MPLS MAN network.

Figure 6-1 2547oDMVPN Deployment

Platforms

Only 7200VXR is supported as the hub router. NPE-G1/G2 is recommended, along with VAM2/VAM2+/VSA modules for encryption. ISRs are recommended as spoke devices. The following images were tested in the lab:

•7200VXR with NPE-G1/G2—12.4(11)T1

•ISRs—12.4(11)T1

Hub and Spoke Communication

The hub and spoke communication is straightforward as it follows the normal P-PE forwarding mechanism. The example below gives the typical configurations for a DMVPN hub router used as a MPLS P device and a DMVPN spoke router used as a MPLS PE device.

Hub11:

!

hostname ngwan-hub11

!

crypto isakmp policy 1

encr 3des

authentication pre-share

crypto isakmp key Cisco123 address 0.0.0.0 0.0.0.0

crypto isakmp keepalive 10 5

!

!

crypto ipsec transform-set T1 esp-3des

mode transport

!

crypto ipsec profile P1

set transform-set T1

!

interface Loopback1

ip address 100.0.250.33 255.255.255.255

ip pim sparse-mode

!

interface Tunnel1

bandwidth 1500

ip address 150.0.0.1 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp map multicast dynamic

ip nhrp network-id 1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf priority 100

load-interval 30

mpls ip

tunnel source 192.168.1.14

tunnel mode gre multipoint

tunnel key 777

tunnel protection ipsec profile P1

!

router ospf 1

mpls traffic-eng router-id Loopback1

mpls traffic-eng area 0

router-id 100.0.250.33

log-adjacency-changes

network 100.0.250.33 0.0.0.0 area 0

network 100.0.0.0 0.0.255.255 area 0

network 150.0.0.0 0.0.0.255 area 0

!

router ospf 100

log-adjacency-changes

network 192.168.1.12 0.0.0.3 area 3

!

ip pim ssm range 1

access-list 1 permit 239.232.0.0 0.0.255.255

Spoke br11:

hostname ngwan-br11

!

ip vrf vpn1

rd 100:10

route-target export 100:110

route-target import 100:110

mdt default 239.232.1.1

mdt data 239.232.1.128 0.0.0.127 threshold 10

!

ip vrf vpn2

rd 100:20

route-target export 100:120

route-target import 100:120

mdt default 239.232.2.1

mdt data 239.232.2.128 0.0.0.127 threshold 10

!

ip multicast-routing

ip multicast-routing vrf vpn1

ip multicast-routing vrf vpn2

!

crypto isakmp policy 1

encr 3des

authentication pre-share

crypto isakmp key Cisco123 address 0.0.0.0 0.0.0.0

crypto isakmp keepalive 10 5

!

!

crypto ipsec transform-set T1 esp-3des

mode transport

!

crypto ipsec profile P1

set transform-set T1

!

interface Loopback1

ip address 100.0.250.35 255.255.255.255

ip pim sparse-mode

!

interface Tunnel1

bandwidth 1500

ip address 150.0.0.2 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp map 150.0.0.1 192.168.1.14

ip nhrp map multicast 192.168.1.14

ip nhrp network-id 1

ip nhrp nhs 150.0.0.1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf priority 0

load-interval 30

mpls ip

qos pre-classify

tunnel source 192.168.1.2

tunnel mode gre multipoint

tunnel key 777

tunnel protection ipsec profile P1

!

interface POS5/0

ip address 192.168.1.2 255.255.255.252

load-interval 30

crc 32

clock source internal

service-policy output wan-edge

!

router ospf 100

log-adjacency-changes

network 192.168.1.0 0.0.0.3 area 3

!

router ospf 1

mpls traffic-eng router-id Loopback1

mpls traffic-eng area 0

router-id 100.0.250.35

log-adjacency-changes

network 100.0.250.35 0.0.0.0 area 0

network 150.0.0.0 0.0.0.255 area 0

network 150.0.1.0 0.0.0.255 area 0

!

router bgp 64512

bgp log-neighbor-changes

neighbor RRs peer-group

neighbor RRs remote-as 64512

neighbor RRs update-source Loopback1

neighbor 100.0.250.9 peer-group RRs

neighbor 100.0.250.10 peer-group RRs

!

address-family ipv4

neighbor 100.0.250.9 activate

neighbor 100.0.250.10 activate

no auto-summary

no synchronization

exit-address-family

!

address-family ipv4 vrf vpn2

redistribute connected

no synchronization

exit-address-family

!

address-family ipv4 vrf vpn1

redistribute connected

no synchronization

exit-address-family

!

ip pim ssm range 1

access-list 1 permit 239.232.0.0 0.0.255.255

ngwan-br11#sh ip bgp vpn vrf vpn1 1.28.0.1

ngwan-br11#sh ip bgp vpnv4 vrf vpn1 1.28.0.1

BGP routing table entry for 100:10:1.28.0.0/30, version 269

Paths: (1 available, best #1, table vpn1)

Not advertised to any peer

Local, imported path from 100:180:1.28.0.0/30

100.0.250.22 (metric 69) from 100.0.250.9 (100.0.250.9)

Origin incomplete, metric 0, localpref 100, valid, internal, best

Extended Community: SoO:100:110 RT:100:110

Originator: 100.0.250.22, Cluster list: 0.0.0.1

mpls labels in/out nolabel/21

ngwan-br11#sh ip cef vrf vpn1 1.28.0.1

1.28.0.0/30, version 12, epoch 0

0 packets, 0 bytes

tag information set

local tag: VPN-route-head

fast tag rewrite with Tu1, 150.0.0.1, tags imposed: {63 21}

via 100.0.250.22, 0 dependencies, recursive

next hop 150.0.0.1, Tunnel1 via 100.0.250.22/32

valid adjacency

tag rewrite with Tu1, 150.0.0.1, tags imposed: {63 21}

ngwan-br11#

ngwan-br11#ping vrf vpn1 1.28.0.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.28.0.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

ngwan-br11#

Notice that there are two labels assigned to the destination address in one of the VRFs within the enterprise MPLS MAN. This is exactly the expected behavior for using MPLS to extend the network segmentation to remote branches. Further test with ping shows the LSP and successfully established.

Spoke-to-Spoke Communication (via Hub)

In branch aggregation scenarios, while most traffic is typically between the hub and the spokes, there is always a requirement for spoke-to-spoke communication. VOIP traffic is a good example of peer-to-peer traffic. In a normal DMVPN scenario this would have been achieved by dynamically creating direct spoke-to-spoke tunnels. While there are obvious advantages to this approach, there are issues as well:

•Depending on the underlying physical connectivity, in certain cases the spoke-to-spoke path may not necessarily be better then the spoke-hub-spoke path which could be a problem for latency sensitive traffic such as VOIP.

•There is a possibility of receiving out-of-order packets as during the initial tunnel setup time the traffic traverses the hub, but once the spoke-to-spoke tunnel is setup it switches it over.

•Depending on the number of spoke-to-spoke tunnels that need to created/maintained simultaneously, this can put scale pressures on the spoke router especially if it is a low-end CPE.

Additionally, the MPLS network requires packets to be label switched all the way between source PEs and destination PEs. Running MPLS over DMVPN tunnels makes the remote branch router a full function PE router, which means label imposition is done in the branch router and label switching must be performed all the way between spokes. This requirement make the direct spoke-spoke communication impossible due to the lack of a label allocation mechanism on the dynamically created spoke-spoke tunnels. However, label switching between spoke PE routers can easily be done if spoke-hub-spoke switching path is implemented. With this approach, the hub router act as a MPLS P router, maintains the LDP neighbor relationship, and exchanges label allocation information with all spoke routers. The hub router label switches the packets in-and-out the mGRE interface between the spokes. Since it is done in the fast path (whether encrypted or not), there should be minimal performance implications other then the increase in the hub traffic.

While this solution breaks the benefit of dynamically building spoke-to-spoke tunnels, it provides a acceptable and often more deterministic path for spoke-to-spoke communications and meets the segmentation requirement. It is a very attractive solution when the large enterprise needs to extend their MPLS-segmented data center or large campus to remote branches.

Configuration Example:

The following example shows the two VPNs in the two remote branches (br11 and br12) communicate to each other via the DMVPN hub router (hub11). The router hub11's configuration is the same as shown in Hub and Spoke Communication. The VPN naming and address scheme, along with the address of hub router and branch routers, are illustrated in Figure 6-1.

Br11:

hostname ngwan-br11

!

ip vrf vpn1

rd 100:10

route-target export 100:110

route-target import 100:110

mdt default 239.232.1.1

mdt data 239.232.1.128 0.0.0.127 threshold 10

!

ip vrf vpn2

rd 100:20

route-target export 100:120

route-target import 100:120

mdt default 239.232.2.1

mdt data 239.232.2.128 0.0.0.127 threshold 10

!

ip multicast-routing

ip multicast-routing vrf vpn1

ip multicast-routing vrf vpn2

!

crypto isakmp policy 1

encr 3des

authentication pre-share

crypto isakmp key Cisco123 address 0.0.0.0 0.0.0.0

crypto isakmp keepalive 10 5

!

!

crypto ipsec transform-set T1 esp-3des

mode transport

!

crypto ipsec profile P1

set transform-set T1

!

interface Loopback1

ip address 100.0.250.35 255.255.255.255

ip pim sparse-mode

!

interface Tunnel1

bandwidth 1500

ip address 150.0.0.2 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp map 150.0.0.1 192.168.1.14

ip nhrp map multicast 192.168.1.14

ip nhrp network-id 1

ip nhrp nhs 150.0.0.1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf priority 0

load-interval 30

mpls ip

qos pre-classify

tunnel source 192.168.1.2

tunnel mode gre multipoint

tunnel key 777

tunnel protection ipsec profile P1

!

interface POS5/0

ip address 192.168.1.2 255.255.255.252

load-interval 30

crc 32

clock source internal

service-policy output wan-edge

!

router ospf 100

log-adjacency-changes

network 192.168.1.0 0.0.0.3 area 3

!

router ospf 1

mpls traffic-eng router-id Loopback1

mpls traffic-eng area 0

router-id 100.0.250.35

log-adjacency-changes

network 100.0.250.35 0.0.0.0 area 0

network 150.0.0.0 0.0.0.255 area 0

network 150.0.1.0 0.0.0.255 area 0

!

router bgp 64512

bgp log-neighbor-changes

neighbor RRs peer-group

neighbor RRs remote-as 64512

neighbor RRs update-source Loopback1

neighbor 100.0.250.9 peer-group RRs

neighbor 100.0.250.10 peer-group RRs

!

address-family ipv4

neighbor 100.0.250.9 activate

neighbor 100.0.250.10 activate

no auto-summary

no synchronization

exit-address-family

!

address-family ipv4 vrf vpn2

redistribute connected

no synchronization

exit-address-family

!

address-family ipv4 vrf vpn1

redistribute connected

no synchronization

exit-address-family

!

ip pim ssm range 1

access-list 1 permit 239.232.0.0 0.0.255.255

Br12:

!

hostname ngwan-br12

ip vrf vpn1

rd 100:10

route-target export 100:110

route-target import 100:110

mdt default 239.232.1.1

mdt data 239.232.1.128 0.0.0.127 threshold 10

!

ip vrf vpn2

rd 100:20

route-target export 100:120

route-target import 100:120

mdt default 239.232.2.1

mdt data 239.232.2.128 0.0.0.127 threshold 10

!

mpls label protocol ldp

!

crypto isakmp policy 1

encr 3des

authentication pre-share

crypto isakmp key Cisco123 address 0.0.0.0 0.0.0.0

crypto isakmp keepalive 10 5

!

!

crypto ipsec transform-set T1 esp-3des

mode transport

!

crypto ipsec profile P1

set transform-set T1

!

interface Loopback1

ip address 100.0.250.36 255.255.255.255

ip pim sparse-mode

!

interface Tunnel1

ip address 150.0.0.3 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp authentication spe

ip nhrp map 150.0.0.1 192.168.1.14

ip nhrp map multicast 192.168.1.14

ip nhrp network-id 1

ip nhrp holdtime 360

ip nhrp nhs 150.0.0.1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf priority 0

load-interval 30

mpls label protocol ldp

mpls ip

qos pre-classify

tunnel source Serial2/0

tunnel mode gre multipoint

tunnel key 777

tunnel protection ipsec profile P1

interface Serial2/0

ip address 192.168.1.6 255.255.255.252

load-interval 30

dsu bandwidth 44210

service-policy output wan-edge

!

router ospf 100

log-adjacency-changes

network 192.168.1.4 0.0.0.3 area 3

!

router ospf 1

mpls traffic-eng router-id Loopback1

mpls traffic-eng area 0

log-adjacency-changes

network 100.0.250.36 0.0.0.0 area 0

network 150.0.0.0 0.0.0.255 area 0

network 150.0.1.0 0.0.0.255 area 0

!

router bgp 64512

bgp log-neighbor-changes

neighbor RRs peer-group

neighbor RRs remote-as 64512

neighbor RRs update-source Loopback1

neighbor 100.0.250.9 peer-group RRs

neighbor 100.0.250.10 peer-group RRs

!

address-family ipv4

neighbor 100.0.250.9 activate

neighbor 100.0.250.10 activate

no auto-summary

no synchronization

exit-address-family

!

address-family vpnv4

neighbor RRs send-community extended

neighbor 100.0.250.9 activate

neighbor 100.0.250.10 activate

exit-address-family

!

address-family ipv4 vrf vpn2

redistribute connected

no synchronization

exit-address-family

!

address-family ipv4 vrf vpn1

redistribute connected

no synchronization

exit-address-family

!

ip pim ssm range 1!

access-list 1 permit 239.232.0.0 0.0.255.255

!

The following command shows the seamless integrating of remote spokes with the Enterprise MPLS network. VPN routes are distributed by RRs in the enterprise MPLS network via MP-iBGP:

ngwan-br11#sh ip bgp vpnv4 vrf vpn1 10.10.2.1

BGP routing table entry for 100:10:10.10.2.0/24, version 3267

Paths: (2 available, best #2, table vpn1)

Not advertised to any peer

Local

100.0.250.36 (metric 133) from 100.0.250.10 (100.0.250.10)

Origin incomplete, metric 0, localpref 100, valid, internal

Extended Community: RT:100:110

Originator: 100.0.250.36, Cluster list: 0.0.0.1

mpls labels in/out nolabel/76

Local

100.0.250.36 (metric 133) from 100.0.250.9 (100.0.250.9)

Origin incomplete, metric 0, localpref 100, valid, internal, best

Extended Community: RT:100:110

Originator: 100.0.250.36, Cluster list: 0.0.0.1

mpls labels in/out nolabel/76

ngwan-br11#

The following command shows two labels are allocated for the VPN routes in spoke routers and hub is in the middle of the LSP:

ngwan-br11#sh ip cef vrf vpn1 10.10.2.1 detail

10.10.2.0/24, version 423, epoch 0

0 packets, 0 bytes

tag information set

local tag: VPN-route-head

fast tag rewrite with Tu1, 150.0.0.1, tags imposed: {78 76}

via 100.0.250.36, 0 dependencies, recursive

next hop 150.0.0.1, Tunnel1 via 100.0.250.36/32

valid adjacency

tag rewrite with Tu1, 150.0.0.1, tags imposed: {78 76}

ngwan-br11#

The following command shows the VPN traffic between spokes are label switched via hub router:

ngwan-br11#traceroute vrf vpn1 10.10.2.1

Type escape sequence to abort.

Tracing the route to 10.10.2.1

1 150.0.0.1 [MPLS: Labels 78/76 Exp 0] 4 msec 0 msec 0 msec

2 10.10.2.1 4 msec * 8 msec

ngwan-br11#

Connecting to the Core MPLS Network

The core MPLS network and the DMVPN-based MPLS network are fully integrated together when the DMVPN hub router act as a MPLS P router. The normal MPLS/LDP configuration applies here when it connects to the enterprise MPLS core networks.

Building Redundancy

Redundancy can be built at various points within the networks:

•Use of multiple routers and HSRP/GLBP with the Enhanced Object Tracking at the branch

•Multiple hub routers at the headend

•Hub connects to multiple core routers

Ideally all three should be used to provide a robust end-to-end connectivity solution. For cost reasons, it may not be feasible to have two routers at every branch; however it should be implemented at least at the large branches when the high-available requirement is a must. While the loss of a spoke router may not be critical to the network, loss of the hub may mean loss of multiple sites and hence more critical. Thus each spoke should be connected to multiple hubs via GRE tunnels which are maintained in active/standby state by controlling the route metrics.

While it may seem desirable to keep both (or all) the hubs as active/active and allow the traffic to be load balanced, we do not see any true advantage in doing so. Keeping the tunnels as active/standby allows the hubs to be better engineered for steady state performance. It also reduces the load on the spoke routers while allowing more deterministic traffic path characteristics.

The various options discussed here will are illustrated using an example.

Example:

In the following example (Figure 6-2), two remote spoke sites br11 and br13 (br13a, br13b) are connected to two hubs (hub11 and hub12). Br11 is considered as a single-tier branch, where it has only one WAN router with two DMVPN tunnels terminated at hub11 and hub12. Hub11 is the primary hub and hub12 is the backup. Br13 is considered as a Dual-tier branch, representing a large branch with two WAN routers with the dual DMVPN tunnel which provides the WAN link redundancy. HSRP with enhanced object tracking is used to provide the network resiliency for the clients on the branch.

Figure 6-2 2547oDMVPN Redundancy

Note The encryption configuration is shown here as an example and standard best practices for IPSec should be followed in an actual deployment.

Below is the configuration example for br11, which use dual tunnels on the same router for the WAN redundancy. OSPF cost is used to tune the routing matrix to select which hub is the primary one.

Spoke B21a:

hostname ngwan-br11

!

crypto isakmp policy 1

encr 3des

authentication pre-share

crypto isakmp key Cisco123 address 0.0.0.0 0.0.0.0

crypto isakmp keepalive 10 5

!

crypto ipsec transform-set T1 esp-3des

mode transport

!

crypto ipsec profile P1

set transform-set T1

!

interface Tunnel1

bandwidth 1500

ip address 150.0.0.2 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp authentication ngwan

ip nhrp map 150.0.0.1 192.168.1.14

ip nhrp map multicast 192.168.1.14

ip nhrp network-id 1

ip nhrp nhs 150.0.0.1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf priority 0

load-interval 30

mpls ip

qos pre-classify

tunnel source 192.168.1.2

tunnel mode gre multipoint

tunnel key 777

tunnel protection ipsec profile P1 shared

!!

interface Tunnel2

ip address 150.0.1.2 255.255.255.0

no ip redirects

ip mtu 1368

ip pim sparse-mode

ip nhrp authentication ngwan

ip nhrp map 150.0.0.1 192.168.1.18

ip nhrp map multicast 192.168.1.18

ip nhrp network-id 2

ip nhrp nhs 150.0.1.1

ip nhrp cache non-authoritative

ip ospf network point-to-multipoint

ip ospf cost 100

ip ospf priority 0

load-interval 30

qos pre-classify

mpls ip

tunnel source POS5/0

tunnel mode gre multipoint

tunnel key 888

tunnel protection ipsec profile P1 shared

!

Spokes Br13a and Br13b:

Spoke Br13a has the DMVPN tunnel connected to Hub11 while br13b's DMVPN tunnel connects to hub12. Br13a is selected as the HSRP active router (higher priority and preemption enabled) and br13b is configured as the standby, This also makes the hub11 as the primary hub and hub12 as the backup hub for this remote site. Working together with the HSRP, Enhanced object tracking is also configured to track two objects, the line protocol on the tunnel interface and the reachability to the hub itself (tunnel destination address). The latter is a more reliable object to track since there are scenarios where the line protocol on the tunnel interface may not go down.

Since the rest of the configuration is similar to other spokes, only the HSRP relevant configuration is shown here:

hostname ngwan-br13a

!

track 1 interface Tunnel1 line-protocol

track 2 ip route 192.168.1.14.255.255.255.252 reachability

!

hostname ngwan-br13a

!

track 1 interface Tunnel1 line-protocol

delay up 50

track 2 ip route 192.168.1.14.255.255.255.252 reachability

delay up 50

!

interface Vlan110

ip vrf forwarding vpn1

ip address 10.10.4.2 255.255.255.0

standby 1 ip 10.10.4.3

standby 1 timers 1 3

standby 1 priority 105

standby 1 preempt

standby 1 track 1 decrement 10

standby 1 track 2 decrement 10

!

hostname ngwan-br13b

!

track 1 interface Tunnel1 line-protocol

delay up 50

track 2 ip route 192.168.1.18.255.255.255.252 reachability

delay up 50

!

interface Vlan110

ip vrf forwarding vpn1

ip address 10.10.4.2 255.255.255.0

standby 1 ip 10.10.4.3

standby 1 timers 1 3

standby 1 preempt

standby 1 track 1 decrement 10

standby 1 track 2 decrement 10

!

Understanding Convergence

We focus on traffic convergence for the hub <-> spoke traffic. As seen in the redundancy section, there are two major backup options available for two types of branch architecture—single-tier branch which having multiple tunnels originating on the same router and dual-tier branch which use two separate routers with HSRP at the branches.

Single-Tier Branches—Backup Tunnel on the Same Router

When the backup tunnel is on the same router, the traffic convergence is primarily dependent on the IGP. By keeping the default timers, following test has conducted to know the network convergence time.

The failure/recovery is simulated by shut/no shut of the link connecting the hub to the SP (doing it on the SP router).

Table 6-1 Convergence When the Primary Tunnel is Down

Iteration

1

2

3

Down

Up

Down

Up

Down

Up

Spoke-to-hub traffic

5.5s

0s

5s

0s

4s

0s

Hub-to-spoke traffic

6s

0s

5s

0s

5s

1s

Table 6-2 Convergence When the Primary Router is Reloaded

Iteration

1

2

3

Down

Up

Down

Up

Down

Up

Spoke-to-hub traffic

1.5s

0s

2s

4s

2s

1s

Hub-to-spoke traffic

15s

0s

1.5s

3s

2s

0.6s

As can be seen tuning down the BGP timers provides a much faster convergence. While tuning down to (1s, 3s) provides the best end-to-end convergence performance, it is not recommended in a scaled environment due to the additional overhead on the hub router. At the very least the performance impact needs to be studied in a scaled environment before tuning it down to such levels.

Dual-Tier Branches—Backup Tunnel on Different Routers

With two WAN routers used in the dual-tier branch, the primary and backup tunnels are configured on two different WAN routers (HSRP enabled). Since the branch routers is actually a MPLS PE device maintaining the MP-iBGP session with RRs, the MP-iBGP convergence time is a big factor when the failover happens. In addition, two other factor need to be considered as well—object tracking detection of absence of the DMVPN tunnel availability and HSRP switchover.

We test the failure as in the previous case by shutting the SP link to the hub1 as well as reloading the primary hub. Two sets of BGP timer are tested: BGP default timer and BGP keepalive timers set to (2s, 6s). Shorter BGP timers like (2s, 6s) are not recommended for large-scale deployments without additional testing and is provided here for comparison only.

Convergence Time When BGP Default Timer is Used

Two scenarios, primary link down/up and primary router reload, have been tested.

Table 6-3 Convergence When the Primary Tunnel is Down

Iteration

1

2

3

Down

Up

Down

Up

Down

Up

Spoke-to-hub traffic

4s

0s

6s

2s

5s

1s

Hub-to-spoke traffic

3s

0s

5s

0s

5s

1s

Table 6-4 Convergence When the Primary Router is Reload

Iteration

1

2

3

Down

Up

Down

Up

Down

Up

Spoke-to-hub traffic

8s

1.5s

10s

1.2s

8s

1.5s

Hub-to-spoke traffic

8s

1s

10s

1.5s

8s

1s

While HSRP provides better redundancy and fast LAN convergence, the convergence on the WAN side is affected by other factors. With object tracking we are watching for the hub's tunnel source interface, which comes up first (in the case of up convergence), but BGP itself takes much longer since it has to wait for the Tunnel itself to come up. Thus even though HSRP has switched over to the original active router (because of preempt), BGP convergence takes longer. This issue can be addressed by delaying the HSRP switchover time when BGP is converging, which can be done by adding the delay statement for the objects that have been tracked. The delay timer implemented in the network needs to adjust based on the BGP convergence time, which needs to be tested in a scenario that's very close to the production network.

From the network design perspective, how the network redundancy is implemented in the remote branch is an integrated part of the overall branch architecture. As shown above, different redundancy approaches yield to different convergence time and which one should be used needs to be evaluated from the overall branch architecture point of view.

Note HSRP timers were kept at 1s for hello and 3s for holdtime in both the cases.

Implementing Multicast

Multicast VPN (MVPN) is the technique used to delivery multicast traffic across the MPLS network for different VPNs (user groups). From a multicast perspective, DMVPN is treated as any other transport media although we do have to account for its multipoint nature in the design.

Assuming that the enterprise MAN MPLS network at the headend is already MVPN enabled, then it is a matter of extending the functionality to the branches. In our example, each VRF is set up with static anycast RP with MSDP enabled, which provides simplicity and redundancy. The RPs typically reside closer to the source and this case the RP is configured in campus CE device at the enterprise MAN data center for each VPN (user group), where the source is connected. All the VPNs in each of the spokes has reachability to the RP and source, so from a VRF perspective the setup looks similar to a normal multicast network.

In the global space mVPN need to be implemented across the entire MPLS network where multicast is required. PIM-SSM or PIM-Bidir are the recommended protocols for the core. Default MDT is used to maintain the control plan traffic and low rate data traffic as well. Data MDT is created automatically when the traffic exceeds the configured threshold. Data MDTs are even more important in the DMVPN network because without them the hub would end up replicating the multicast traffic for all the spokes that are attached to it irrespective of whether they have receivers or not. With Data MDTs the spokes would only join the specific (S,G) if they had receivers for it. This saves CPU resources on the headend device and bandwidth at the hub and the spokes. Two conditions need to be met for the Data MDTs to be initiated:

•The traffic threshold needs to be low enough to enable (set it to 1kbps for almost instantaneous initiation).

•(S,G) entries need to exist within the VRF.

Additionally, PIM NBMA mode needs to be configured on the mGRE interface. This creates the spoke specific entry in the Multicast Output Interface List (OIL).

Caveats:

•CSCse05807 identifies problems with multicast forwarding—received mvpn traffic is process switched on ISRs. This problem is observed when the ISRs are used as a PE (irrespective of using 2547oDMVPN).

Configuration Example:

The following is built on our earlier example (Figure 6-2) and only the multicast-relevant configurations are shown here. The VPN and address scheme is illustrated in Figure 6-1. The multicast traffic is delivered from the campus to remote branches.

Hub11:

Hostname ngwan-hub11

!

ip multicast-routing

!

interface Tunnel1

ip address 150.0.0.1 255.255.255.0

ip pim nbma-mode

ip pim sparse-mode

!

ip pim ssm range 1

access-list 1 permit 239.232.0.0 0.0.255.255

!

Br11:

ip vrf vpn1

!

mdt default 239.232.2.1

mdt data 239.232.2.128 0.0.0.127 threshold 10

!

ip multicast-routing

ip multicast-routing vrf vpn1

!

interface Tunnel1

ip address 150.0.0.2 255.255.255.0

no ip redirects

ip pim sparse-mode

!

interface Loopback1

ip address 100.0.250.35 255.255.255.255

ip pim sparse-mode

!

!

router bgp 64512

!

address-family vpnv4

neighbor RRs send-community extended

neighbor 100.0.250.9 activate

neighbor 100.0.250.10 activate

exit-address-family

!

ip pim spt-threshold infinity

ip pim ssm range 1

ip pim vrf vpn1 rp-address 1.28.103.1

access-list 1 permit 239.232.0.0 0.0.255.255

!

Below are the show commands that illustrate the packet flow from the source PE in enterprise MPLS campus to the receiving PEs in the remote branches.

Let's first check the mroute table in both global and VPN space on the campus PE connecting to the source:

•Ensure that the MDT switchover threshold is set to the lowest value to enable the data MDTs.

Implementing QoS

A basic assumption of this implementation is that the enterprise is getting a Layer 3 VPN service from a provider. Thus the level of QoS service from a provider becomes important. Typically, a service with 3-5 classes of service can be expected. We do not focus on SP service in this design guide as this has been discussed extensively in the Consumer Guidance WP (http://www.cisco.com/application/pdf/en/us/guest/netsol/ns465/c654/cdccont_0900aecd80375d78.pdf). We focus on the aspects of QoS that are within enterprise control, primarily on WAN Edge QoS at the DMVPN headend and the branches.

QoS policies required on the headend include queuing, shaping, selective dropping, and link-efficiency policies in the outbound direction of the WAN link. Traffic is assumed to be correctly classified and marked (at Layer 3) before it ingresses the headend router. At the headend the expectation is that a interface with high link speed is used (DS3/OC3/GE range). At these speeds, link-efficiency policies such as LFI and cRTP are not required. The Enterprise QoS SRND recommends 5-11 classes at the WAN edge. The choice would be dependent on the existing core QoS deployment. We use a 8-class model in our example. The typical bandwidth allocation for a 8-class model is shown in Figure 6-3 (from the SRND).

Figure 6-3 8 Class QoS Model

At the branches, the PE could be configured to map the COS to DSCP, but in our example we assume that the packets are already marked with the appropriate DSCP. If the branches have slow/medium speed links (<T1) then a 3-5 class model is recommended. One option could be to match the model used by the SP providing the Layer 3 VPN service. We assume that the branches have higher speed links (>T1) and implement a 8-class model as well (similar to the hub).

Overall the WAN QoS recommendation made in the Enterprise QoS SRND remain for 2547oDMVPN as well. This is because the labeled packets are encapsulated in the GRE header and at the outgoing interface the packet looks like a normal IP packet which can be treated under existing guidelines.

For packets going from hub to spoke and vice-versa, as shown in Figure 6-4, the DSCP/TOS from original IP packet is copied to the MPLS EXP (automatic IP Precedence to EXP mapping) as well as to outer headers DSCP/TOS field.

Figure 6-4 Headers for QoS—Hub and Spoke Traffic

Packets traverse the hub in the case of spoke-to-spoke communication. In this case the source branch behavior remains same as above. At the hub, the GRE header is stripped off before a forwarding decision is taken, which in this case requires adding another GRE header before forwarding it back out to the destination branch. As shown in Figure 6-5, the EXP gets copied to the outgoing GRE IP header as Precedence (3 bits only). Thus the outgoing policy on the hub should account for DSCP as well as Precedence.

Figure 6-5 Headers for QoS—Spoke to Spoke Traffic

The original IP headers marking are always preserved in either case.

Note At the hub, since all the traffic is placed inside the same GRE tunnel, per spoke QoS is not supported.

Example:

Hub1 has a OC3 ATM connection to the SP and spoke B21a has a high speed Ethernet connection. LLQ is used for Voice and Interactive Video traffic. The rest of the classes are provided a bandwidth percentage. DSCP-based WRED is enabled for both Critical Data and Bulk Data.

Hub1:

class-map match-any Bulk-Data

match ip dscp af11

match ip dscp af12

match ip precedence 1

class-map match-any Interactive-Video

match ip dscp af41

match ip dscp af41

match ip precedence 4

class-map match-any Network-Control

match ip dscp cs6

match ip dscp cs2

match ip precedence 6

class-map match-any Critical-Data

match ip dscp af21

match ip dscp af22

match ip precedence 2

class-map match-any Call-Signaling

match ip dscp cs3

match ip dscp af31

match ip precedence 3

class-map match-any Voice

match ip dscp ef

match ip precedence 5

class-map match-any Scavenger

match ip dscp cs1

!

policy-map WAN-EDGE

class Interactive-Video

priority percent 15

class Call-Signaling

bandwidth percent 5

class Network-Control

bandwidth percent 5

class Critical-Data

bandwidth percent 27

random-detect dscp-based

class Bulk-Data

bandwidth percent 4

random-detect dscp-based

class Scavenger

bandwidth percent 1

class Voice

priority percent 18

class class-default

bandwidth percent 25

random-detect

!

interface ATM5/0

ip address 135.0.13.2 255.255.255.252

pvc 1/1

vbr-nrt 44209 44209

service-policy output WAN-EDGE

max-reserved-bandwidth 100

Note Scavenger traffic is mapped to Bulk Data at the hub for spoke-to-spoke communication.

Spoke B21a:

class-map match-all Bulk-Data

match ip dscp af11 af12

class-map match-all Interactive-Video

match ip dscp af41 af42

class-map match-any Network-Control

match ip dscp cs6

match ip dscp cs2

class-map match-all Critical-Data

match ip dscp af21 af22

class-map match-any Call-Signaling

match ip dscp cs3

match ip dscp af31

class-map match-all Voice

match ip dscp ef

class-map match-all Scavenger

match ip dscp cs1

!

policy-map WAN-EDGE

class Voice

priority percent 18

class Interactive-Video

priority percent 15

class Call-Signaling

bandwidth percent 5

class Network-Control

bandwidth percent 5

class Critical-Data

bandwidth percent 27

random-detect dscp-based

class Bulk-Data

bandwidth percent 4

random-detect dscp-based

class Scavenger

bandwidth percent 1

class class-default

bandwidth percent 25

random-detect

!

interface FastEthernet1/1

description To SP

ip address 135.0.5.1 255.255.255.252

max-reserved-bandwidth 100

service-policy output WAN-EDGE

Note QoS policy is applied to the outgoing physical interface only. No policy is required on the mGRE interface.

MTU Issues

As with any tunneled implementation, MTU size can become an issue. This becomes particular acute with MVPN over 2547oDMVPN when the underlying service is a Layer 3 VPN service from a provider and mVPN is used to delivery the multicast packets. As can be seen in Figure 6-6, at the hub the original IP multicast packet is encapsulated in a multicast GRE header (MTI) which is encapsulated in the unicast GRE header (DMVPN) before being sent to the SP (with appropriate Layer 2 header added). This means that the original IP packet ends up with an additional overhead of at least 48 bytes (without encryption).

Figure 6-6 2547oDMVPN—Multicast Packet Overhead

In the case of unicast, things are a little better. The original IP packet has a MPLS label attached to it before being encapsulated into the unicast GRE (DMVPN). As shown in Figure 6-7, the additional overhead can be expected to be at least 28 bytes (without encryption).

Figure 6-7 2547oDMVPN—Unicast Packet Overhead

The safest MTU (the worst case MTU) for tunnel interface to avoid fragmentation is 1400 bytes. Taking into consideration that each MPLS label is 4 bytes, the safest MTU on 2547oDVMPN tunnel is 1392 Bytes for unicast traffic and 1368 for multicast traffic.

"ip tcp adjust-mss <value>" can also be used to inform the end device to use the correct MSS for TCP transmissions. The MSS must be set to a value that equals the interface MTU minus the size of IP, TCP, GRE, and MPLS headers.

The GRE interface can also be configured with "mpls mtu" which sets the MTU for the labeled packets. MPLS MTU is derived by adding (label stack x 4bytes) to the interface MTU.

Voice and VRFs

Typically voice traffic has no dependency on the network type since they are just transported as IP packets and require correct QoS behavior applied to them. An exception is when routers are used as gateways for voice services because a lot of voice features and protocols deployed at the branches are not VRF aware (for example, SRST, CME, etc. Thus just getting the voice traffic in a VRF could be a challenge. This is apart from larger issues of having the voice in a VRF; while you can have the IP phones within a VRF, other services such as softphones VT advantage may be in a different VRF. There are challenges in implementing Inter-VRF IP communications. These are not discussed here as its part of the larger virtualization architecture issue. The current recommendation is to keep voice within the global space especially at the branches. At the hub they could remain in the global space or would have to be placed within its own VRF. We look at both options, getting the voice in the VRF at the branch as well keeping it in the global table at the branch.

Voice in a VRF at the Branch

If we need to put the voice in the VRF and still want to use voice features such as CME, then the only way to currently do this is by having two separate routers at the branch. The branch edge router still has a voice VRF configured but treats it like any other VRF. It has a second router (such as a low end ISR) connected to its voice VRF VLAN. The second router, as shown in Figure 6-8, has all the phones attached to it. Since it requires two routers at every such branch, this can be a expensive proposition.

Figure 6-8 2547oDMVPN—Voice in a VRF at the Branch

Example:

Branch 21 has the voice VRF configured on B21a. B21a has 21b connected to it within VRF red-voice. B21b provides the connection support for IP/Pots phones within the branch.

Voice Global at the Branch

If we choose to keep the voice global at the branch then a single router would suffice. The voice VLAN is connected to the branch router but remains in the global space. It is carried across the same GRE that is used to carry labeled packets but as normal IPv4 traffic. At the hub router, it remains in the global space. It can be forwarded to the next hop router on a global VLAN or can be forwarded to core MPLS PE where it can be placed in its own VRF.

Note To get the voice traffic routed, a routing protocol needs to be used over the GRE tunnels. This potentially has a scaleability impact on the DMVPN setup (relying solely on MP-BGP vs. a combination of MP-BGP and IGP for route distribution).

Example:

B22a in our example is a PE which uses MP-BGP for all the VRFs except voice. Voice traffic exists in global space and hence voice VRF is not defined on it. It runs OSPF with the hub. At the hub the traffic remains in the global space but it has a OSPF peering with PE5. On PE5 the traffic is placed within voice VRF as shown in Figure 6-9.

Scale Considerations

The 2547oDMVPN hub can support at least 500 remote spokes as we have tested this scenario in the lab with a simulated customer-representative environment. OSPF is used as the routing protocol between DMVPN hub and spoke. Hence the following scalability numbers apply to this setup; these numbers will satisfy many of the customer scaling requirements:

Solution Caveats Summary

2547oDMVPN provides a scalable solution for both greenfield virtualization deployments and established DMVPN deployments moving towards virtualization. The following list summarizes the caveats for the deployment model discussed here:

•No direct spoke-to-spoke tunnels can be established. Spoke-to-spoke communication has to happen through the hub.

•For multicast ensure that PIM-NBMA mode is configured on the tunnel interface.